Inner-outer finned heat transfer tubes

ABSTRACT

Inner-outer finned heat transfer tubes are provided for use in heat exchangers. The heat transfer tubes include a central core body surrounded by a tubular member. The central core has a plurality of integral elongated fins extending along the length of the core and projecting outwardly therefrom. The tubular member has a plurality of externally protruding fins on the outer surface thereof.

United States Patent Kim [451 Oct. 10, 1972 [54] INN ER-OUTER FINNED HEAT TRANSFER TUBES [72] Inventor: Sung Chul Kim, Des Plaines, Ill.

[73] Assignee: International Telephone & Telegraph Corporation, New York, NY.

[22] Filed: Jan. 2, 197 0 [2]] Appl. No.: 287

[52] US. Cl ..165/179, 165/181 [51] Int. Cl. ..F28f 1/42 [58] Field of Search ..165/179, 183, 184, 181,182

[56] References Cited UNITED STATES PATENTS 2,281,206 4/1942 Schoen ..165/79;

Primary Examiner-Charles Sukalo Attorney-C. Cornell Remsen, Jr., Walter J. Baum, Percy P. Lantzy, J. Warren Whitesel, Delbert P. Warner and James B. Raden [57] ABSTRACT Inner-outer finned heat transfer tubes are provided for use in heat exchangers. The heat transfer tubes include a central core body surrounded by a tubular member. The central core has a plurality of integral elongated fins extending along the length of the core and projecting outwardly therefrom. The tubular member has a plurality of externally protruding fins on the outer surface thereof.

8 Claim, 8 Drawing Figures FATENTED 10 3,696, 863

sum 1 BF 2 ATTORNEY INNER-OUTER FINNED HEAT TRANSFER TUBES This invention relates to heat exchangers and to the finned heat transfer tubes employed therein. More particularly, it relates to inner-outer finned heat transfer tubes for use in heat exchangers and to methods for producing these tubes.

Heat exchangers are devices for transferring heat from one fluid to another without allowing the fluids to mix. It is to be recognized that the term fluid, as employed herein, is meant to include liquids, gases, vapors and mixtures thereof.

ln tube type heat exchangers, one fluid flows internally to the tubes and the other fluid flows externally thereto within an outer shell. Generally, heat transfer between the fluids is accomplished by means of convection from the hot fluid, whether internal or external to the tubes, to the solid surface of the tubes, then by conduction through this solid material and convection from the other surface of thesolid pipe to the cold fluid. The rate of heat transfer in such systems, known as the overall coefficient of heat transfer, is the reciprocal of the sum of the individual thermal resistances encountered in series along the path of heat flow.

The rate at which heat may be transferred from the tube surface to the main body of fluid is known as the film coefficient. When the fluid is circulated artifically over the heat-transfer surface, the value of the film coefficient is governed by the velocity, temperature difference and physical properties of the fluid, and by the size, shape, arrangement and nature of the surface.

It has been a continuing problem to maximize the heat transfer efficiency of tube type heat exchangers. To accomplish this, a combination of factors must be taken into consideration. For example, in attaining maximum heat transfer efficiency, the overall heat transfer coefficients of the unit must be maximized for a given surface area and fluid temperature difference. This can be accomplished by improving film coefficients on both sides of the tubing.

As a practical matter in designing tube type heat exchangers, it is desirable to promote turbulence in fluid. flow within the outer shell external to the tubing since this factor will tend to increase the heat-transfer coefficients. Also, the outer and inner surface area of the heat transfer tubes should be increased to promote an increased rate of heat transfer. Further, in many instances, it is very desirable for the heat transfer tubes to have a low outside to inside surface area ratio, optimally approaching unity, whereby heat transfer is markedly improved. This low ratio is particularly desirable in applications where the internal and external fluids normally produce poor film coefficients, as in the case of gas-to-gas applications.

From a commercial standpoint, it would be very desirable to provide tube type heat exchangers which achieve the herein described objects and features and, in addition, are less costly to produce and more economical in operation than heretofore proposed heat exchangers. Additionally, it would be of great value if the heat exchangers could be produced in a more compact form with substantially reduced size.

If heat transfer tubes are produced having a unitary metallic body, considerable precision "tooling is required. Consequently, such tubes have tended to cost more than is desirable considering the number of such tubes that are required for use in heat exchangers. Further, the performance of such tubes as regards heat transfer efficiency has not been sufficient to justify the increased cost.

Additionally, problems are encountered in utilizing heat transfer tubes having internal fins within a plain external tube, particularly for operations wherein the fluids used have poor film coefficients such as gas-togas operations. Such tubes do not provide adequate film coefficients on both sides of the tubing to be effective for such application.

Furthermore, heat transfer tubes generally have not possessed sufficient total surface area per lineal foot; nor have the outside to inside surface area ratios been low enough to provide sufficiently improved heat transfer. This is particularly important for applications wherein both fluids normally produce poor film coefficients, as is the case with air-to-air applications.

For example, known heat transfer tubes do not provide adequate performance ingas-to-gas heat exchangers utilizing a refrigeration cycle to cool compressed air for moisture control Such systems are commercially important since they are employed for conditioning compressed air in manufacturing plants and the like, and for use with pneumatic tools and controls. Consequently, it would be very advantageous to provide new and improved heat transfer tubes which can be used effectively in such heat exchange systems having air-to-air, air-to-refrigerant, or gas-to-gas applications, because of increased surface area, improved film coefficients on both sides of the tubing and lower outside to inside surface area ratio.

A further disadvantage of the previous finned heat transfer tubes is that they have not provided sufficiently increased heat transfer coefficients. Therefore, it has not been practical to reduce the number of tubes employed in heat exchangers and correspondingly to reduce the size of the heat exchangers in which they have been employed. Accordingly, it would be highly desirable and economically beneficial to provide new and improved heat transfer tubes which, because of their improved heat transfer efficiencies, enable the production of more compact, less costly heat exchange units.

Accordingly, an object of the present invention is to provide new and improved heat exchangers of the tube type.

Another object is to provide new and improved shell and tube type heat exchangers employing unique innerouter finned heat transfer tubes. In this connection, these heat exchangers are constructed in a more economical manner, are more compact, and have improved heat transfer coefficients.

Another object is to provide new and improved bimetal inner-outer finned heat transfer tubes for use in heat exchangers which are more efficient in operation and can be produced more efficiently and more economically.

Another object is to provide unique innerouter tinned heat transfer tubes which have increased surface area and improved film coefficients on both sides of the tubing.

A further object is to provide heat transfer tubes which possess increased heat transfer efficiencies when the fluids employed have poor film coefficients. In this regard, an object is to provide heat transfer tubes having a low outside to inside surface area ratio whereby heat transfer is improved markedly in applications wherein the fluids employed normally produce poor film coefficients.

A still further object is to provide new and improved methods for producing heat transfertubes. In this connection, anobjectis to provide efficient and economical methods for producing inner-outer finned heat transfer tubes which are more efficient in operation.

Yet another object is to provide methods for producing unique inner-outer finned heat transfer tubes employing low-cost general purpose tools, rather than high cost precision extrusion tooling used hitherto in producing finned heat transfer tubes.

In keeping'with an aspect of the invention, these and other objects are accomplished by providing an innerouter finned heat transfer tube constructed of an aluminum extruded inner core or spline having integral elongated fins extending along the length of the core and projecting outwardly from and substantially perpendicular to the surface of the core and a copper externally finned tube. The inner aluminum core is crimp connected to the externally finned tube to produce a rugged, unitary heat transfer tube structure having increased heat transfer efficiencies.

Crimping of the aluminum core with the inner fins into the externally finned tube serves a dual purpose. First, the crimping operation locks the inner core within. the externally finned tube. Second, the crimping action on the previously externally finned outer surface causes distortion and bending of the external fins. These distorted and twisted fins increase the turbulence and fluid flow between the fins, thus improving the heat transfer. Preferably, more crimping locations are used than there are fins on the inner spline. This reduces the degree of distortion of the tubing and external fins to the most desired level, resulting in more effective use of the fins.

The unique inner-outer finned heat transfer tubes of the present invention provide substantially increased heat transfer coefficients and greatly improved film coefficients. Accordingly, these inventive heat transfer tubes enable the production of heat exchangers having greatly reduced size and cost for any given heat transfer situation.

The abovementioned and other features and objects of this invention and the manner of obtaining them will become more apparent, and the invention will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-section view which shows the end of a solid metallic core rod as it appears at the start of a manufacturing process;

FIG. 2 shows a cross-section of the end of the same core rod after it has been formed, as by being extruded or drawn through a die;

FIG. 3 is a cross-section end view which shows a metal tube or sheath as it appears at the start of a manufacturing process;

FIG. 4 shows a perspective view of the tube of FIG. 3 after an initial external fin forming step;

FIG. 5 shows the finished heat transfer tube includv FIG. 4 andthe finned central core of FIG. 2;

FIG. 6 is a fragmentary expanded view of the encircled area 6 of FIG. 5;

FIG. 7 is a cross-section view taken along line 7-7 of FIG. 5 which shows the crimped bimetal inner-outer finned heat transfer tube; and

FIG. 8 is a perspective view which shows a heat exchanger employing the inner-outer finned heat transfer tubing of FIG. 6.

To produce the inventive inner-outer finned heat transfer tube, a bar of metal suitable for forming an internal core member is extruded or drawn through a die to provide a generally circular cross-section 10 with a specific diameter. For example, asreceived, the metallic bar stock might be nominally circular in cross-section and approximately three-quarters of an inch in diameter, but its circumference may be irregular with any diameter varying by, perhaps, a sixteenth of an inch as compared with any other diameter. After the extrusion, the diameter could be five-eights of an inch, and

the cross-section is perfectly circularwithin acceptable tolerances.

Suitable metals for use in producing the core member are aluminum, zinc, magnesium and the like; although, other metals could be used depending on the specific proposed use.

The circular cross-section, extruded rod 10, FIG. 1, is then formed into an elongated core member 12 by economical processes such as the process of extrusion. The core 12 is formed having a solid central body or rod 14 with a plurality of elongated fins l6 integral with and extending along the length of the centralbody 14. The fins 16 project outwardly from and substantially perpendicular to the surfaceof the centralbody 14 along a radial longitudinal plane lying in the principal longitudinal axis of the central body 14. In the illustrated embodiment of FIG. 2, the core 12 has seven equally spaced elongated fins 16 and, correspondingly, between these fins 16 a series of seven open spaces18 are formed. It will be understood that the number of fins on the core 12 may be varied as desired to afford the most advantageous ratio of inside to outside heat transfer areas.

As shown in FIG. 3, an elongated tubular member 20 is provided having a plain, cylindrical surface. The tubular member 20 is initially subjected to processing to produce a plurality of integral upstanding fins 21 which project outwardly about the outer circumference of the tubular member 20 and lie in a substantially radial circumferential plane perpendicular to the principal axis of the tube 20.

As shown in FIG. 4, the upstanding fins 21 are annular in shape and extend perpendicularly outward from the outer surface of the tube 20 in a thread-like arrangement around the circumference of the tube 20. However, it will be understood that the upstanding fins 21 may be produced in a variety of shapes and configurations such as, for example, disconnected disc-like or circular shaped fins.

The manner of forming the upstanding fins 21 is not crucial. For example, they could be formed on a screwforming machine somewhat similar to the manner of threading a bolt. However, I prefer to roll the tube 20 over a fiat die surface in a manner such that the fins 21 are formed having annular shapes with a slight helical twist circumferentially disposed along the longitudinal axis of the tube 20.

The overall inside dimensions of the tubular member should be slightly larger than the outside dimensions of the radial fins 16 of the core member 12. Thus, the core 12 slips easily inside the tube 20. However, as illustrated in FIGS. 5 and 7, the core 12 should fit snugly within the tube 20 when it is inserted therein.

As a practical matter, there is inevitably a somewhat irregular contact between the inner surfaces of the tube 20 and the elongated inner fins 16. However, if effective and efficient heat transfer or flow is to be attained between the fins 16 and the tube 20, the contact between the fins 16 and the tube 20 must be a nearly perfect metal to metal contact. This contact must be so perfect and firm that there is no substantial space between the two surfaces where there might be a layer of heat insulating gas or air. In effect, a virtually gas tight seal must be formed between the edges of the fins 16 and the inside surface of the tube 20.

In the present invention the metal to metal contact of the tube 20 and the fins 16 is attained in such a way that the contact will be maintained even though the tube 20 and the core 12 may be subjected to unequal amounts of radial expansion. The way in which this contact is attained also adapts the tube for bending without destroying the metal to metal contact of these surfaces. To attain such contact the parts are so formed and assembled that there is a constant resilient force acting to maintain such contact.

As illustrated in FIGS. 5 and 7, the inner core or spline 12 is assembled and locked within the externally finned tube 20 by inserting the core 12 within the tube 20 and subjecting the assembly to inward crimping forces at a plurality of positions along the circumference of the outer surface of the tube 20. This crimp locking operation is performed so that a nearly perfect mated core-to-tube combination is provided.

More specifically, FIG. 6 shows an enlarged fragment of the finished tube 26 taken from the encircled area 6 of FIG. 5. Initially, as illustrated in FIG. 4, the tube 20 has a plurality of upstanding annular fins 21 which project outwardly about the outer circumference of the tube 20 and extend outwardly along a radial plane thereof. After crimping, as for example, with a hydraulic tool having somewhat dome shaped fingers,

the annular fins 21 are reshaped to form a plurality of circumferentially disposed depressed regions 24 with somewhat shelf-like surfaces 22 extending away from the radial plane. The depressed regions 24, which are generally semi-circular in cross-section, lie in planes which are approximately parallel to the outer surface of the tube 20. The depressed regions 24 on each fin are generally aligned with corresponding depressed regions on the other fins, as best illustrated in FIG. 5, to form grooves or flutes 25 which extend along the length of the tube 20. Preferably, these grooves 25 follow a slow helical or spiral pattern around the tube 20 which provides improved heat transfer characteristics to the tube 20.

For convenience of expression, this configuration of FIG. 6 is hereinafter termed the externally protruding, inwardly arcuate fins 21 (or, more simply, the external fins 21) having intervening grooves 25.

It is to be noted that more crimping locations are employed than the number of elongated inner fins 16 on the core 12. This promotes reduced distortion of the external fins 21 resulting in more effective flow area between adjacent fins 22. However,a degree of bending and twisting of the fins 21 is highly desirable since the bent or twisted external fins 21 will promote increased turbulence in fluid flow between adjacent fins. When the tubes 26 are employed in a heat exchanger, this increased turbulence causes greatly improved heat transfer efficiency. Accordingly, it should be further noted that the crimping force or pressure exerted on the externally finned tube surface to form the externally protruding inwardly arcuate fins 21 and to lock the core 12 within the tube 20, also causes controlled bending and twisting of the fins 21.

The tube 20, including the external fins 21, is normally constructed from copper. However, other materials may be employed to meet specific conditions that are to be encountered in use.

The materials to be used for the core 12 and the tube 20 may be rather freely selected from materials having the best and most desirable characteristics insofar as corrosion resistance, heat transfer characteristics and cost may be concerned.

In many instances, it is desirable to employ a spiral path for the fluid that is to pass through the heat exchange tube 26. Accordingly, in a further embodiment of the present invention, the core 12 is formed having a spiral twist whereby spiral flow of fluid through the tube is attained.

In production, the twisted core 12 is formed so that the radial fins 16 have a spiral form of desired lead. This may be accomplished readily by application of twisting forces to the core 12. The twisted core 12 is then inserted endwise into an externally finned tube 20 such as that shown in FIG. 4. Then, crimping forces are applied to the tube 20 in the manner described hereinbefore to produce a unique inner-outer finned heat transfer tube having a spiral or twisted inner core.

In the use of the inner-outer finned heat transfer tube with a spiral or twisted inner core, the fluid flows within the tube 26 along the several spiral paths 28 defined between the twisted fins 16. When the fluid is a mixture of liquid and gaseous refrigerant and the rate of refrigerant flow through the tube 26 is relatively high, the liquid refrigerant is separated by centrifugal force so as to flow along the inner surfaces of the tube 26, while the gaseous component of the refrigerant mixture flows along the inner portions of the space.

From the foregoing, the operation of the inventive heat transfer tubes now should be apparent. In greater detail, a heat exchanger, shown generally as 30 in FIG. 8, is provided having an outer shell 32. A plurality of heat transfer tubes, corresponding to tube 26, are positioned within shell 32. Four ports 34, 36, 38 and 40 are provided about the surface of shell 32. Ports 34 and 36 are, respectively, the inlet and outlet ports for the fluid flowing through the heat transfer tubes 26. Ports 38 and 40 are, respectively, the inlet and outlet ports for the fluid flowing within shell 32, external to the tubes 26 through a baffled flow path, indicated generally by arrow 42.

In operation, fluids such as air or refrigerant are individually introduced into the exchanger 30 through inlet ports 34 and 38 by suitable means such as compressors, pumps, blowers, and the like, not shown. The fluid entering port 34 then circulates through the tube clearance holes 28 of tubes 26 and is evacuated therefrom through outlet port 36. The fluid entering port 38 flows through baffle path 42, thus flowing at about right angles to the tubes 26. This fluid comes into contact with the externally finned surface of tube 26 and the fins 21, creating turbulence in the fluid flow which serves to more completely bring fluid into contact with the tube surface and thus to improve the heat transfer efficiency of the unit. The fluid continues to flow through path 42 being exposed to heat transfer effects throughout the flow path. Eventually, the condi-. tioned fluid reaches outlet port 40 where it is forced out of the unit.

In the case of a gas-to-gas operation, assuming that the temperature of the gas flowing through the tubes 26 is lower than the the temperature of the gas flowing through path 42, a pressure drop is created about the outer surface of the tubes 26. This pressure drop will cause the gas in area 42 to permeate down into the external fins 21 of tubes 26. Thus, more turbulent flow of gas through the finned outer surface of tubes 26 is achieved resulting in improved heat exchange. The cooled gas flowing through path 42 is then forced out of the exchanger 30, for example, as conditioned compressed air.

The heat transfer tubes of the present invention have been tested to demonstrate their superior heat transfer capabilities. The results of these tests showed a 100 percent increase in overall heat transfer coefficients of the inventive tubes as compared with plain unfinned heat transfer tubes (i.e., no internal or external fins). Further comparative tests have demonstrated that the inner-outer finned tubes of the present invention have overall heat transfer coefficiencies 67 percent greater than plain external heat transfer tubes having inner fins.

While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention.

lclaim:

1. A bimetal inner-outer finned heat transfer tube comprising an elongated core locked within a surrounding elongated tube, said core having a central body with a plurality of elongated fins integral with and extending along the length of said core, said elongated fins projecting outwardly from and perpendicular to the surface of said core, said elongated fins contacting the inner surface of said tube, said tube having a plurality of externally protruding, inwardly arcuate fins which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes, said externally protruding fins being spaced about the outer circumference of said tube with intervening grooves, said grooves extending along the length of said tube and following a slow helical pattern around said tube.

2. An inner-outer finned heat transfer tube comprising an elongated tube having incorporated therein an elongated core, said core having a central body with a plurality of integral elongated fins projecting outwardly therefrom, and said tube having a plurality of externally protruding fins on the outer surface thereof which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes.

3. The inner-outer finned heat transfer tube of claim 2 wherein said externally protruding fins are spaced about the circumference of said tube with intervening grooves, said fins being inwardly arcuate in shape.

4. The inner-outer finned heat transfer tube of claim 2 ,wherein said externally protruding fins are spaced about the circumference of said tube with intervening grooves, said grooves being aligned in a slow helical configuration around said tube and extending along the length of said tube.

5. The inner-outer finned heat transfer tube of claim 2 wherein said elongated core is formed having a spiral twist thereto whereby spiral flow of fluid through said tube is provided.

6. A heat exchanger having incorporated therein a plurality of bimetallic inner-outer finned heat transfer tubes, each of said inner-outer finned heat transfer tubes comprising an elongated core locked within a surrounding elongated tube, said core having a central body with a pluralityof elongated fins integral with and extending along the length of said central body, said elongated fins projecting outwardly from and perpendicular to the surface of said central body, said elongated fins contacting the inner surface of said tube, said tube having a plurality of externally protruding, inwardly arcuate fins which are disposed in planes extending transversely of said'tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes,

said externally protruding fins being spaced about the outer circumference of said tube with intervening grooves, said grooves extending along the length of said tube and following a slow helical pattern around said tube.

7. The heat exchanger of claim 6 wherein said grooves of said heat transfer tubes follow a slow helical pattern around said tube.

8. A heat exchanger having incorporated therein a plurality of inner-outer finned heat transfer tubes, each of said inner-outer finned heat transfer tubes comprising an elongated tube having incorporated therein an elongated core, said core having a central body with a plurality of integral elongated fins projecting outwardly therefrom, said elongated fins contacting the inner surface of said tube, and said tube having a plurality of externallyprotruding fins on the outer surface thereof which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes. 

1. A bimetal inner-outer finned heat transfer tube comprising an elongated core locked within a surrounding elongated tube, said core having a central body with a plurality of elongated fins integral with and extending along the length of said core, said elongated fins projecting outwardly from and perpendicular to the surface of said core, said elongated fins contacting the inner surface of said tube, said tube having a plurality of externally protruding, inwardly arcuate fins which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes, said externally protruding fins being spaced about the outer circumference of said tube with intervening grooves, said grooves extending along the length of said tube and following a slow helical pattern around said tube.
 2. An inner-outer finned heat transfer tube comprising an elongated tube having incorporated therein an elongated core, said core having a central body with a plurality of integral elongated fins projecting outwardly therefrom, and said tube having a plurality of externally protruding fins on the outer surface thereof which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes.
 3. The inner-outer finned heat transfer tube of claim 2 wherein said externally protruding fins are spaced about the circumference of said tube with intervening grooves, said fins being inwardly arcuate in shape.
 4. The inner-outer finned heat transfer tube of claim 2 wherein said externally protruding fins are spaced about the circumference of said tube with intervening grooves, said grooves being aligned in a slow helical configuration around said tube and extending along the length of said tube.
 5. The inner-outer finned heat transfer tube of claim 2 wherein said elongated core is formed having a spiral twist thereto whereby spiral flow of fluid through said tube is provided.
 6. A heat exchanger having incorporated therein a plurality of bimetallic inner-outer finned heat transfer tubes, each of said inner-outer finned heat transfer tubes comprising an elongated core locked within a surrounding elongated tube, said core having a central body with a plurality of elongated fins integral with and extending along the length of said central body, said elongated fins projecting outwardly from and perpendicular to the surface of said central body, said elongated fins contacting the inner surface of said tube, said tube having a plurality of externally protruding, inwardly arcuate fins which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes, said externally protruding fins being spaced about the outer circumference of said tube with intervening grooves, said grooves extending along the length of said tube and following a slow helical pattern around said tube.
 7. The heat exchanger of claim 6 wherein said grooves of said heat transfer tubes follow a slow helical pattern around said tube.
 8. A heat exchanger having incorporated therein a plurality of inner-outer finned heat transfer tubes, each of said inner-outer finned heat transfer tubes comprising an elongated tube having incorporated therein an elongated core, said core having a central body with a plurality of integral elongated fins projecting outwardly therefrom, said elongated fins contacting the inner surface of said tube, and said tube having a plurality of externally protruding fins on the outer surface thereof which are disposed in planes extending transversely of said tube and at generally right angles to the longitudinal axis of said tube, the externally protruding fins in any of said transverse planes being aligned longitudinally of said tube in spaced, generally parallel relationship to the externally protruding fins in any other of said transverse planes. 